WHAT IS THE BEST DISTAL LEVEL OF ARTHRODESIS IN LUMBAR FUSION IN PATIENTS WITH ADOLESCENT IDIOPATHIC SCOLIOSIS: L3 OR L4?

ABSTRACT Objective To evaluate coronal alignment in patients with idiopathic adolescent scoliosis with structured lumbar curves submitted to surgical treatment by comparing coronal alignment in the group fusion up to L3 and the group fusion up to L4. Methods Retrospective cohort study. We evaluated patients submitted to surgical treatment with arthrodesis of the lumbar curve with high density of screws with at least 6 months of follow-up. Radiographically, coronal alignment, shoulder height and functional outcome were analyzed through SRS30 questionnaire. Results A total of 25 patients were analyzed, of which 23 were female and 2 were male, with a mean age of 15.2 years (12 to 29 years) at the time of surgery. The patients were divided into two groups. Group A, n = 15: Distal level of fusion in L3 and Group B, n = 10: distal level of fusion in L4. There was no statistically significant difference between Groups A and B when compared to coronal alignment (balanced vs. unbalanced). However, when compared with the coronal alignment (CA) values, lower values of CA were observed in Group A, with statistical significance. No difference was observed between Groups A and B with respect to the SRS30 questionnaire. Conclusions Patients with idiopathic adolescent scoliosis submitted to arthrodesis of the lumbar curve have a better coronal alignment when the distal fusion level is L3. Level of evidence III; Comparative Retrospective Study (based on prospectively collected data).


INTRODUCTION
The definition of the arthrodesis level is the most important individual factor in the postoperative outcome of corrective idiopathic scoliosis surgery. 1 Historically, fusion criteria have been evolving over the years, accompanied by changes in spinal correction techniques and instrumentation. 2 At the beginning of scoliosis surgery, during the period of correction with plaster, Risser proposed arthrodesis up to the vertebra parallel to the ground following the application of plaster. 3 In the 1970s, at the beginning of vertebral instrumentation, Harrington proposed that arthrodesis should extend one level above the proximal Cobb and two levels below the distal Cobb, provided that it was in the stable zone (between the foramina of S1). 4 Later, in the modern era of spine surgery, Moe 5 established the neutral vertebra (NV) as the boundary of the arthrodesis and, subsequently in a retrospective study, King reported better results when the arthrodesis extended to the stable vertebra (SV). 6 More recently, Lenke popularized the concept of the touched vertebra (TV) as the appropriate arthrodesis level for some curves. The term substantially touched vertebra (STV) was later defined as the first vertebra touched by the medial sacral line since touching the pedicle. 7 At the same time, Suk proposed a classification that defines the distal level of the lumbar arthrodesis according to the rotation and translation of L3 in tilt radiographs, 1 although there is no consensus around the ideal distal level.
All these criteria were established in order to decrease the number of arthrodesed levels, which is even more important when the arthrodesis extends to the lumbar curve. 8,9 Although some studies report a higher index of lumbar pain and greater disc degeneration the more distal the arthrodesis, some studies report greater coronal imbalance with shorter arthrodeses. 7,[10][11][12][13] The objective of this study is to compare the clinical and radiographical results of patients submitted to posterior arthrodesis stopping at L3 or L4 and to compare the criteria established in the literature to define the distal arthrodesis level (Suk 1 and stable vertebra) with those established by the author.

METHODS
This is a retrospective study based on prospectively collected data. Following approval by the local institutional review board (IRB -82768218.9.0000.0023), the medical records and imaging examinations of patients submitted to surgical treatment for adolescent idiopathic scoliosis (AIS) in a single center, with at least 6 months of follow-up, were evaluated.
Patients who underwent posterior approach deformity correction with inclusion of the lumbar curve (below L2 with lumbar modifier B or C) were included, that is, patients whose lumbar curves were considered structured and were included in the arthrodesis. Patients with no lumbar curve (lumbar modifier A) or for whom the lumbar curve was not included (selective thoracic arthrodesis) were excluded. Patients with other deformities (neuromuscular, congenital, and syndromic) and those who underwent anterior approach surgery were also excluded.
All patients were operated by the same surgeon in a single center using the same surgical correction technique with high density of screws and direct vertebral derotation (DVD). 1,14 The patients with the distal level of the arthrodesis of the lumbar curve in L3 (GROUP L3) were compared with the patients with the distal level of the arthrodesis in L4 (GROUP L4). The criterion equilibrados). Sin embargo, en comparación con los valores de alineación coronal (AC), se observaron valores de AC más bajos en el grupo A, con significación estadística. No se observó diferencia entre los grupos A y B con respecto al cuestionario SRS30. Conclusiones: Los pacientes con escoliosis idiopática del adolescente sometidos a artrodesis de la curva lumbar tienen mejor alineación coronal cuando el nivel distal de fusión es L3. Nivel de evidencia III, Estudio Retrospectivo Comparativo (basado en datos obtenidos prospectivamente).
Descriptores: Escoliosis; Resultado del Tratamiento; Calidad de Vida; Fusión Vertebral. used was defined by the assisting physician, whose philosophy was to always stop as proximally as possible in the lumbar curve, trying to stop at L3 even if it was not touched by the medial sacral line (TV-1). The cases where the arthrodesis stopped at L4 were those with great apical vertebral translation (AVT) and with L4 being the TV-1 (Figure 1).

Clinical and radiographical evaluation
The clinical evaluation was conducted using the SRS 30 questionnaire (Scoliosis Research Society) in the preoperative period (PRE), at the first postoperative visit (IPO), and the last postoperative follow-up (LPO). These questionnaires are already regularly filled out by the patients and are included in their medical records.
The radiographical evaluation was conducted using the following parameters: Cobb angle of the proximal thoracic curve (Co-bbTp), Cobb of the principle thoracic curve (CobbTP), Cobb of the thoracolumbar/lumbar curve (CobbTL/L) in the standing and tilt radiographs. The AVT (apical vertebral translation) and AVR (apical vertebral rotation) of the thoracic curves (AVT T) and L/TL (AVT L), which denote, respectively, the translation and rotation of the apical vertebra, were defined. The coronal alignment (CA) (distance of the C7 plumb line to the medial sacral line [MSL] in cm), the angulation of L4 (AL4) (angle of L4 in relation to the horizontal), and the height of the shoulders (AO) (difference in height between the apices of the coracoid processes, in cm) were measured. [15][16][17] (Figures 2-4) The individuals analyzed were divided into balanced (B) and non-balanced (NB) in all the evaluations (PRE, IPO, and LPO), balanced individuals considered to be those with CA of up to 2 cm. All measurements were taken by the same examiner using Surgimap Spine software (Nemaris Inc., New York, NY). All curves were classified according to Lenke 17 and Suk 1 and the fusion criteria of these authors were compared with the option chosen by the surgeon.
Criterion for distal fusion of the stable vertebra: stop at the vertebra bisected by the medial sacral line (MSL). 6 Suk's criterion for distal fusion: stop at L3 when it exceeds the medial sacral line in the right lateral inclination and rotation is less than Nash-Moe 2 in the left lateral inclination. Otherwise, the fusion should be extended to L4.
Author's criterion for distal fusion: Always try to stop at L3, even if it is not touched at the medial sacral line (TV-1). Stop at L4 only when L3 is TV-2, i.e., L4 must be TV-1.

Statistical analysis
The entire statistical analysis was conducted using SPSS software (version 24.0, IBM SPSS, Inc.). All data were presented as means and medians, standard deviation, and confidence interval (95%). Data distribution was tested by applying the Shapiro-Wilk test. For   comparisons between means, when the distribution was identified as parametric, the Student's t-test for independent samples was used. When the data were non-parametric, the Mann-Whitney test was used. The categorical variables were presented by frequency as absolute numbers and ratios. The analysis of the distribution of the categorical variables was conducted by applying the Chi--square or Fisher's exact test (frequency less than five). We also used Cochran's Q test for the comparison of ratios in related samples with more than two categories, Pearson's correlation analysis test, and Friedman's two-way analysis of variance by ranks in related samples. All tests were applied considering a significance level of 5% and a confidence interval of 95%.
In terms of the distal level of the arthrodesis, fusion extended to L3 (GROUP 3) in 15 (60%) patients and to L4 (GROUP 4) in 10 (40%) patients.

Radiographical assessment
The mean Cobb values in the PRE, IPO, and LPO are shown in Table 1. The percentage of correction of the TL/L curve in GROUP L3 was 62% and in GROUP L4 was 66%. There was no statistical difference between the groups.
As regards coronal alignment (CA), in the preoperative period (PRE) 15 patients (60%) were non-balanced (NB), while 10 (40%) were balanced (B) In the immediate postoperative period (IPO). The ratio did not get worse, but remained the same and improved over the long term, 15 (60%) NB and 10 (40%) B, however in the final assessment there was significant improvement of this parameter with 18 (78%) B and 7 (28%) NB (p=0.038 according to Cochran's Q test for related samples. When we evaluated the patients in GROUP L3 separately, there was a tendency towards improvement in the CA from PRE to LPO and in GROUP L4 a tendency towards worsening ( Figure 5).
The CA was also evaluated quantitatively. There was a statistical difference in CA when the LPO measurements were compared between the two groups. (Table 2) The values of AL4 are shown in Table 3. There was no statistical difference between the groups.
No difference was observed between the preoperative height of the shoulders and AVT measurements between the two groups. (Tables 4 and 5) When the patients were divided by distal fusion level indicated, we observed the following: using the author's criteria, the last instrumented vertebra was L3 in 15 patients (60%) and L4 in 10 patients (40%) and using the criteria of Suk, 1 of all the patients analyzed, only 10 (40%) should stop at L3. When we evaluated the criterion of the stable vertebra, we observed that no patient should have the last vertebra instrumented in L3, with 2 patients (8%) with fusion to L4 and 21 patients (92%) to L5. (Table 6)

Clinical Assessment
Of the 25 patients included in the study, 12 answered the SRS-30 quality of life questionnaire completely. After proper analysis, we observed that among these patients there was a statistically significant improvement in the appearance. There was no influence from the treatment on the remaining domains. (Table 7) There were no statistical differences between the groups in terms of the quality of life questionnaire parameters.  Figure 5. Percentage of Balanced patients in the two groups. In GROUP L3 there was a tendency towards improvement from the PRE to the LPO and in GROUP L4 a tendency towards worsening.    Table 6. Distal level of arthrodesis according to the criteria of Suk, of the stable vertebra, and of the author.

DISCUSSION
The objective of surgical treatment for AIS is to achieve a compensated trunk with the lowest number of arthrodesed vertebrae. 18 This concept becomes even more important when the correction involves fusion of the lumbar curve. Some authors report a higher incidence of lumbar pain and disc degeneration in arthrodeses that extend distally to L3. 19,20 Although more robust evidence of the correlation between extension of the arthrodesis and long-term functional results is still lacking in the literature, the choice of L3 instead of L4 during a fusion of the lumbar curve is the goal of most spine surgeons. 18 However, the risk of decompensation and a lower angular correction can theoretically be increased with this type of approach.
This study showed that the patients submitted to arthrodesis up to L3 had improved CA, while the patients with arthrodesis up to L4 had  a tendency towards worsening. This outcome may be associated with the presence of one more mobile segment distal to the arthrodesis that allows the trunk to be compensated. Another interesting factor was the worsening in the IPO and then improvement in the LPO in both groups, showing that an early radiographical assessment has little importance in relation to trunk balance. Lee also showed greater decompensation of the trunk in patients with arthrodesis to L4 as compared to L3. However, in this study, in spite of the large case series, this difference was not statistically significant. 21 In relation to the angular correction, there was no difference in the percentage of lumbar curve and AL4 correction. The same results were also reported by Lee, although he warned of a possibly lower correction when the last vertebra is L5. 21 As regards the clinical results, the patients generally presented significant improvement in the domain of personal appearance and of the overall questionnaire. However, there was no difference when groups 3 and 4 were compared. The lack of a clinical difference in the short-term assessment of these two groups of patients is corroborated by other studies in the literature. 18,21 When we evaluated the percentage of patients submitted to arthrodesis to L3 in relation to the most popular criteria in the literature, we observed that the author's criteria permit shorter arthrodeses in a significant percentage of patients with similar clinical and radiographical results. This demonstrates that Suk's criteria and the stable vertebra criteria may be overestimating the need for arthrodesis to L4.
This study has a series of limitations: first, for its retrospective design even though the data was collected prospectively; additionally, for the small case series and short follow-up time; finally, for the low proportion of patients who filled out the quality of life questionnaires correctly, impacting the clinical assessment of this group of patients.

CONCLUSION
Stopping arthrodesis in L3, which presents similar clinical and radiographical results in the short term, may be a good option in order to save levels in the lumbar region. It can be performed even in cases where the other criteria indicate arthrodesis up to L4, such as when L3 is the TV or even the TV-1.